Various measuring methods using the Brillouin scattering phenomenon and the Rayleigh scattering phenomenon of an optical fiber are known (e.g. WO 2006/001071). An example is a distribution type pressure sensor, which uses the Brillouin frequency shift and Rayleigh frequency shift generated by applying strain (pressure) to an optical fiber. These frequency shifts depend on the strain applied to the optical fiber, hence the applied pressure can be measured by measuring the frequency shift amount.
This pressure measuring technique using an optical fiber can be applied to measuring the volumetric changes of an object. For example, porous sandstone, the volume of which changes between before and after liquid is filled, is one application field of this pressure measuring technique. Lately a technique to store carbon dioxide (CO2) underground is in-development as a countermeasure to global warming, and this pressure measuring technique can contribute to constructing a system to monitor the state of storing CO2 in sandstone when CO2 is stored underground, and a system to monitor the dynamic stability and safety of a caprock layer (e.g. pelitic rock), which is an upper layer above the sandstone layer.
However a method for accurately detecting, for example, the volumetric changes of sandstone constituted by an unknown composition that exists underground has not yet been proposed. It is possible to detect the changes of pressure at a spot if an electric pressure sensor is used. However it is impossible to specify whether these pressure changes are caused by the pressurized entry of fluid into the sandstone or by the volumetric changes of the sandstone.